The generation of acoustic waves in the surface of a body of piezoelectric material has been described, inter alia, in the IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-17 No. 11, November 1969, at page 845, entitled "The Generation and Propagation of Acoustic Surface Waves at Microwave Frequencies". On a per wavelength basis the transmission loss for acoustic waves is less than that for corresponding electromagnetic waves in TEM or waveguide media.
Transducers for launching and recovering surface acoustic waves have been developed. One form of transducer is that known as the interdigital transducer. The efficiency of the interdigital transducer is enhanced when two or more pairs of such transducers are used in an interlaced structure, with all the input transducers connected in parallel electrically, likewise the output transducers. With large numbers of pairs of transducers in an interlaced structure extremely low loss devices can be fabricated.
Conventional electrical transmission line filters comprise circuit elements consisting of capacitive and inductive obstacles, short circuit and open circuit terminations and similar devices. Capacitive and inductive obstacles in a waveguide or transmission line exhibit their properties because of the stored energy (electric or magnetic) associated with the evanescent modes which they excite. However, in surface acoustic wave devices obstacles to an acoustic wave excite many new modes which are non-evanescent and thus the effect is one of mode conversion.